Hydrocarbon fluids, such as crude oil, crude oil emulsions, oilfield condensate, petroleum residua and even refined fuels often contain a variety of mercaptans, including mercaptans of relatively low molecular weight. In the drilling, production, transport, storage, and processing of hydrocarbon stocks, the mercaptans encountered can cause many problems ranging from malodors to metal corrosion. Because of the volatility of mercaptans of relatively low molecular weight (for example, methyl mercaptan, CH3SH, ethyl mercaptan, CH3CH2SH and propyl mercaptan, CH3CH2CH2SH), they tend to evolve into vapor spaces, where their offensive odors create problems in and around storage areas and throughout pipelines and shipping systems used for transporting the hydrocarbon. Mercaptans in crudes, naphthas and gasoline streams present safety concerns and corrosion hazards.
Various additives have been employed in efforts to alleviate these problems. For example, choline or choline hydroxide has been found to alleviate hydrogen sulfide (H2S) evolution and to scavenge mercaptans. See, for example, U.S. Pat. No. 4,594,147 to Roof et al., U.S. Pat. No. 4,867,865 to Roof and U.S. Pat. No. 5,183,560 to Roof et al. However, choline and choline hydroxide are not well suited for many uses and media, such as in crude oil. Although choline and choline hydroxide might scavenge mercaptans in such media, they also form a volatile and malodorous by-product with the sulfur compounds indigenous to such media. Accordingly, the use of choline and choline hydroxide to control odors associated with light weight mercaptans is self-defeating in media such as crude oil. Thus, the cited patents to Roof and Roof, et al. fail to address this problem and instead describe the use of choline or choline hydroxide in the more refined fuel oils.
European application 0 538 819 A3 to Roof et al. describes the use of oil-soluble quaternary ammonium compounds of the formula:
to scavenge various sulfur compounds, including mercaptans, from certain oils, especially high boiling, heavy residual fuels. These compounds, prepared under anhydrous conditions, are what are described herein as “internal ions”; i.e., the positive charge on the nitrogen and the negative charge on the oxygen result in overall electrically neutral compounds without the presence of counter ions such as halides. The European application stresses the significance of the oil solubility of these compounds, noting that they are more oil soluble than choline base and so disperse through the oil being treated more thoroughly to decrease the concentration of undesirable sulfur compounds more effectively. Nevertheless, the compositions of the European application suffer from certain disadvantages. For example, compositions that are produced in higher yields, yet still at low cost, and that reduce mercaptan concentrations more effectively are still desired.
U.S. Pat. Nos. 5,840,177 and 6,013,175 relate to methods for scavenging mercaptans in hydrocarbon fluids using quaternary ammonium hydroxides. Other chemistries and methods for removing mercaptans from hydrocarbons include caustic (NaOH solutions) and cobalt with caustic (Merox™ process of UOP, Merichem processes).
A quick removal of these smelly mercaptan species would be desirable in both oilfield and refinery operations. Many conventional mercaptan scavengers remove and/or react with the mercaptans present only slowly. There is a continuing need in the liquid fuel industry for treating liquid hydrocarbon stocks and wet or dry gas mixtures containing mercaptans using new compositions and methods that would scavenge mercaptans quickly.